The building fabric -external walls and floors (Eco-Refurbishment)

Cavity walls

In a typical inter-war suburban home around 35 to 45 per cent of its heat can be lost through its walls. Since about 1930, most houses in the UK have cavity wall construction. Approximately 75 per cent of houses in the UK still have not taken up the benefits of cavity insulation. It is claimed that the heat transfer through a cavity wall can be reduced by up to 63 per cent. Energy saving of between 19 and 35 per cent is possible if there is also loft insulation and the home is heated to the same standard as previously.

The process

Three materials meet the British Standard for cavity insulation. They are

• Rock wool

• Glass wool

• Expanded polystyrene beads.

Some installers include urea-formaldehyde foam which sets within the cavity into a meringue-like consistency. In the past there have been serious problems associated with off-gassing and providing pathways for damp around openings. It may be that these problems have been solved, but negative memories have a long shelf-life.

Insulation is blown into the cavity through holes about 20 mm diameter and 1.35 m apart.

Rock wool or glass wool are the most common insulants being stable over the lifetime of the building and impervious to moisture damage. In most cases cavities are about 50 mm wide, which means that a wall U-value W/m2K can be achieved with:

• both leaves of brick 0.57

• outer leaf brick, inner leaf dense concrete block 0.54

• outer leaf brick, inner leaf lightweight concrete block 0.49

These values fall short of the 2002 Building Regulations standard for existing cavity wall houses of 0.45 W/m2K. To reach this standard would require either internal dry lining insulation or external insulation as described below.

It is worth noting that the National Insulation Association claims that cavity insulation could, for most houses, achieve U-values between 0.38 and 0.43 W/m2K. Before installing the insulation it is advisable that all wiring within the cavity is checked for compliance with current regulations by an accredited electrician. Underfloor ventilation should be safeguarded by sleeving air vents where necessary. After the operation all holes must be filled with matching mortar. The loose wool is blown into the cavity until it is tightly packed and will therefore not be subject to settlement. In semi-detached or terraced houses a brush barrier is placed in the cavity at the junction with an adjacent property.

It is impossible to give an accurate estimate of the payback time for this investment since it depends on location, the nature of the building fabric, the hours of heating and the comfort temperature set by the inhabitants. However, it is almost certainly within the range of 5-10 years.

As a rough guide, the cost of the operation at late 2002 prices should be:

3 bedroom detached house


3 bedroom semi-detached house


3 bed mid terrace house


3 bedroom bungalow


Quality control

In the past, there have been serious problems of quality control sometimes degenerating into criminal fraud. A number of steps should be taken to ensure a quality installation.

• Ensure that the insulation material being used and process employed are covered by a British Board of Agrement (BBA) certificate.

• Use installers whose work is regularly inspected and approved by the BBA.

• Confirm that the installation is guaranteed for 25 years under the Insulation Guarantee Agency, the government approved guarantee scheme. The guarantee covers defects in materials and workmanship which will be rectified without charge. The guarantee can be transferred to subsequent owners of the property.

• Choose an installer who is a member of the National Cavity Insulation Association (NCIA). The association should provide a list of approved installers.

As a precautionary footnote, in many older cavity wall homes it has been found that the wall ties connecting the two leaves have corroded in many cases to the point of disintegration. Before installing cavity insulation it would be wise to have a specialist check the state of the cavity so that, if necessary, new wall ties can be inserted.

Solid walls

In the UK, a large proportion of homes are either solid stone or 225 mm solid brickwork or concrete block. Solid walls were being built in some areas up to 1939. It is likely that a good number of home owners do not know whether they have a solid or cavity wall. This can be checked at window or door reveals. A plastered cavity wall should measure overall about 290 mm, whereas a solid brick wall will come to roughly 240 mm. If the exterior is rendered a further 18-25 mm should be added.

There are two ways of insulating solid external walls:

• by cladding the external face with insulation or ‘overcladding’

• by fixing insulation to the internal face called ‘dry lining’.

It is the first option which has the greater potential to achieve significant reductions in heat loss through the fabric. It also involves adjustments to several features of the fabric. However, one big advantage is that it can be applied without the house needing to be vacated.


Two methods of external insulation are available

• Fixing insulation board to the external face and finishing with a waterproof render

• A thick render coat containing an insulant sprayed or trowelled on to the wall surface.

The best results are obtained by the first option, insulation board fixed to the brickwork or blockwork. If a house is rendered it may be necessary to make good the surface to provide a sound fixing for the insulation board. If the backing wall substrate is sound and offers a clean, regular surface the insulation board can be fixed with adhesive. Otherwise mechanical fixing is necessary. For the operation to make a significant impact on heating bills, it will usually be necessary to have at least 100 mm of insulation. This will have implications for several external features which will need to be modified:

• If the roof eaves do not overhang sufficiently to accommodate the extra wall width, there will have to be modifications, for example by having an extended gutter which forms a closer to the insulation.

• To avoid cold bridges at openings, the insulation should also cover window and door reveals, which means that windows and doors may need to be replaced or modified. A cold bridge or thermal bridge is where there is discontinuity in the insulation resulting in a section of wall being colder than its surroundings. This attracts condensation and staining.

• Rainwater pipes and soil drain pipes will also have to be modified or boxed-in by the insulation. As a compromise it may be possible to insert thinner insulation boards behind fall pipes, but this does run the risk of cold bridging. The position of open and back inlet gullies may have to be modified. This can be very costly, and a compromise would be to insert a ‘swan neck’ section to the fall pipes at their base to bring them back to the face of the masonry wall. This will mean modifying the insulation at this point.

• Attention must be paid to the damp proof course (dpc) to ensure that the insulation backing does not cause dampness due to capillary action bypassing the dpc.

Quality control

The body that is recognized by government as the national trade association for the external cladding industry is the Insulated Render and Cladding Association (INCA). An installer (Figure 4.1) who is a member of the association is covered by an Agrement Certificate and the association offers a 10-year guarantee for the product and workmanship. At the same time installer members of INCA offer certificated systems which have been granted a 25-30 year life. This ambiguity needs to be resolved (Insulated Render and Cladding Association Ltd, PO Box 12, Haslemere, Surrey GU27 3HA;; Tel: 01428 654011.)

Some individual manufacturers offer a service which is covered by an Agrement Certificate, such as Pittsburgh Corning which describes itself as a ‘Total Quality organization, working under ISO 9002′ (the international standard). Its main product is ‘FOAMGLAS’ which is a cellular glass insulant. As indicated earlier, cellular glass is CFC- and HCFC-free, is impervious to any form of moisture, fireproof, dimensionally stable, gives off no toxic fumes, has high compressive strength and is proof against rot, vermin and insect infestation (

An alternative is StoTherm External Wall Insulation Systems which uses EPS insulation guaranteed CFC- HCFC-free. It offers insurance-backed cover for up to 10 years on adhesion, weather-proofing, cracking and colour fading (

Members of INCA fixing insulation and applying trowelled final coat of render.

Figure 4.1 Members of INCA fixing insulation and applying trowelled final coat of render.

As a guide to the thermal efficiency of various thicknesses of insulation applied externally to a 140 mm solid masonry wall plastered internally should be a U-value in W/m2K of:

65 mm insulation

0.45 (minimum in N.Ireland)

90 mm insulation

0.35 (Current Building Regulations England and Wales)

105 mm insulation

0.3 (pending Building Regulations in England and current in Scotland)

120 mm insulation

0.27 (Republic of Ireland)

External finish

It is necessary for the insulation boards to receive a finishing coat. In the case of most insulants the finish should offer total waterproofing. A polymer-based render is the most reliable in this respect. This is an adhesive render with an alkali-resistant glass fibre mesh as reinforcement. Applied in one or two coats offering a choice of finishes, for example:

External cladding being installed to solid wall homes, Penwith Housing Association.

Figure 4.2 External cladding being installed to solid wall homes, Penwith Housing Association. 

• pebble dash or spar dash

• textured renders in a range of colours

• roughcast, also called harling or wet cast.

It is also possible to use cladding which include

• Lightweight natural stone aggregate

• Brick

• Tile, e.g. terracotta

• Weatherboarding

Traditional render systems around 25 mm thick consist of a base coat and top coat to comply with BS 5262. These systems normally require a metal lath carrier and can receive a mineral or synthetic finish.

Penwith Housing Association in Penzance, Cornwall took over the housing stock of the borough, much of which was solid masonry construction. It took the decision to overclad using the M.R. Swisslab system (Figures 4.2 and 4.3). This system offers a comprehensive service, including aluminium flashings to cover the insulation at eaves level and extension cills for windows.

The insulation is 70 mm of phenolic foam finished with a polymer cement render. Situated on the south west tip of the UK, these houses are exposed to extreme weather.

Cost guidance

It is only possible to offer general guidance since each home is a special case. According to the Insulated Render and Cladding Association an overcladding project costs between £45 and £65/m2 as at the end of 2002. For a semi-detached house with about 80 m2 of wall area, this translates to between £3500 and £5500.


• There is a significant improvement in comfort levels throughout the whole house.

• The walls of the building are protected from weathering, ensuring a longer life.

• There should be absolute protection from penetration by damp.

• The incidence of condensation is reduced to near zero.

• It allows the fabric of the home to act as a heat store – a warmth accumulator.

• It stabilizes the structure, preventing cracking due to differential thermal expansion.


Figure 4.3

M.R. Swisslab system application details. Courtesy of Alumasc Exterior Buildings Products Ltd.

Figure 4.3

M.R. Swisslab system application details. Courtesy of Alumasc Exterior Buildings Products Ltd.

• Space heating bills can be reduced by up to 50 per cent.

• The increase in property value as a result of the upgrading usually more than offsets the cost.

• There is normally a significant improvement in appearance.

• The operation can be undertaken without the need to vacate the property.

• There is a significant reduction in carbon dioxide emissions. Government estimates suggest that, over the lifetime of the building, one tonne of CO2 is saved for every square metre of 50 mm thick insulation

A company which claims to be the world’s leading manufacturer of the combination of external insulation and finishes is Dryvit Systems Inc. Dryvit UK Ltd is a wholly owned subsidiary. Its standard system is called ‘Outsulation’ (Figure 4.4). Its ‘Roxsulation’ system incorporates mineral wool insulation. It is BBA approved. Care needs to be taken that it conforms to the latest version of Part L of the Building Regulations.

Dryvit external cladding system installed at Baggy House, UK.

Figure 4.4 Dryvit external cladding system installed at Baggy House, UK.

Sprayed insulation

Sprayed application of a two-part polyurethane foam is another possibility. ISL Renotherm claims that 45 mm of foam can bring an external wall up a U-value of 35 W/m2K which is the current Building Regulation standard.

Internal insulation (dry lining)

Where it is not practical to adopt cavity or external insulation, the only option is to revert to dry lining. The dilemma is that it reduces internal space. To bring a 140 mm solid external wall up the Buildings Regulations standard would require at least 90 mm of insulation with a plasterboard finish. A suitable insulant is cellular glass fixed to the wall mechanically.

Dry lining using Foamglas P+R board with plasterboard or plaster finish. Courtesy of Pittsburgh Corning (UK) Ltd.

Figure 4.5 Dry lining using Foamglas P+R board with plasterboard or plaster finish. Courtesy of Pittsburgh Corning (UK) Ltd.

The finish is either plasterboard with a skim coat of plaster or plaster applied to metal lathing. There are consequences to using this system, such as the relocation of skirtings and electrical sockets. There is also the risk of cold bridging if the insulation is not continued around the reveals to openings. This could involve the replacement of external doors and windows. However this is one instance where the best can be the enemy of the good and compromise is reasonable. The recommended U-value for dry-lined external walls is 0.45 W/m2K (Figure 4.5).

Party walls could benefit from dry lining treatment, not necessarily to the thickness of external walls. Not only will this conserve warmth, it would also reduce noise transmission.

A typical 19th century terraced house could benefit from a combination of overcladding and dry lining. The front of the house, which is often at the back or the pavement, would not be suitable for overcladding, making dry lining the only option. However, the rear, which is often L-shaped in plan, could receive overcladding treatment. This is best undertaken on a whole street basis if possible to avoid discontinuity and to realize the benefits of economy of scale.


Suspended floors with underfloor ventilation are a prime source of heat loss. For ground floors and basements the recommended U- value is 0.25 W/m2K.

Floor insulation options.

Figure 4.6 Floor insulation options.

The most common method is to insert the insulation between the floor joists. If there is insufficient crawl space below the joists there is no alternative but to remove the floor boards or sheet covering to achieve a meaningful degree of insulation. The easiest method is to lay plastic netting over the joists and then fill to floor board level with a mineral fibre insulant. Alternatively, battens and a tray can be fixed to the joists allowing a loose fill insulant to be used. Another option is to simply fix battens to the bottom of the joists and use a rigid polystyrene or cellular glass insulant.

Skirting boards should be checked for draughts. If there is an underfloor void it is surprising how much cold air can penetrate the habitable space due to an inadequate seal between skirtings and floorboards (Figure 4.6).

For solid concrete floors there is no alternative but to place the insulation material on top of the slab. To realize a meaningful improvement in thermal efficiency will necessitate raising the floor level by at least 80 mm using a rigid insulant, such as cellular glass. Ideally the floor covering should be removed exposing the screed. It may be necessary to make good the screed with a sand and cement mix to achieve a smooth, level surface. An insulant with high compressive strength such as ‘Foamglas floorboard’ is then laid on the screed. A polyethylene vapour barrier must be laid over the insulation and beneath a chipboard subfloor. The finish is optional.

Where it is not feasible to raise the floor level by 80 mm or so, a compromise expedient is to lay 5 mm cork tiles. Cork has good insulation properties.

Windows and doors

Many home owners elect to replace windows as much for cosmetic as thermal reasons. It is an expensive operation and, in terms of saved energy, the payback time could be up to 15 years. However, this may be more than offset by the increase in the value of the property. This will especially be the case when houses will require an energy rating at the point of sale.

Swedish windows installed (a) in improvement works to flat in Chelmsford, UK; and (b) during the eco-refurbishment of a house in Cumbria, UK.

Figure 4.7 Swedish windows installed (a) in improvement works to flat in Chelmsford, UK; and (b) during the eco-refurbishment of a house in Cumbria, UK. 

Under the 2002 Building Regulations replacement windows must conform to Part L of the Building Regulations unless there are compelling reasons for a waiver, for example in the case of listed buildings. These state that windows, doors and rooflights in a timber or PVCu frame must have a maximum U-value of 2.00W/m2K. It is also advisable to have a minimum of a 12 mm gap between the panes of the double glazing. On this basis double glazing with argon-gas filling and soft low-E coating meets the standard.

If metal frames are necessary perhaps because of the character of the house, or its listed status, the maximum U-value is 2.2 W/m2K. In this case triple glazing with air filling and hard low-E coating is the required specification, plus the fact that thermal breaks must be included in the frames to reduce heat loss through the metal.

A high quality double-glazed window is marketed by Construction Resources. Frames are constructed from the heartwood of slow-growing Scandinavian fir trees that are 120-150 years old.

Timber is the preferred choice for window frames. Whilst softwoods gained a poor reputation in the 1960s and 1970s they are now mostly more reliable. Hardwoods should be from an authenticated sustainable source, preferably native to the UK. PVC frames are ecologically damaging, especially since the manufacturing process involves the use of chlorine.

Help lines

Grants are available for installing insulation and draught proofing from the UK Government’s Warm Front scheme. Grants up to £1500 are available to households with children under 16 and pregnant women who are on income-related or disability benefit. Grants of up to £2500 are available to anyone aged over 60 who is on income-related benefit.In the Yorkshire and Humberside, Eastern and East Midlands areas the Warm Front scheme is administered by TXU

Pensioners are entitled to a winter fuel payment of £200. Information on; Tel: 08459 151515.

The charity Age Concern offers a ‘Help with Heating’ fact sheet (; Tel: 0800 00 99 66).

Help the Aged provides a fact sheet ‘Keep out the Cold’ (; Tel: 0808 800 6565).

Main points

• Check if external walls are cavity construction and if so whether they already have received insulation.

• If not, install cavity fill insulation such as rock wool, glass wool or polystyrene beads, first checking cavity for corroded wall ties or blockages.

• Use an accredited installer such as a member of the National Cavity Insulation Association.

• Check guarantee.

• In the case of solid masonry walls, the options are overcladding or dry lining, or a combination of both.

• For overcladding it is advisable to use a contractor who is a member of the Insulated Render and Cladding Association.

• The maximum possible thickness of insulation should be applied, preferably 120 mm if circumstances allow. The marginal cost of going for the best is soon repaid in lower heating bills.

• External finishes should be watertight and conform to British Standard 5262. There is a wide choice of finishes.

• If the property is listed or in a conservation area it will be necessary to consult the local Planning Department, since there is likely to be a significant change in appearance.

• Internal dry lining should give a solid wall a U-value of at least 0.45 W/m2K.

• Well-insulated floors give good value in terms of insulation and comfort. If floor boards are being replaced the opportunity should not be missed to fill the joist spaces with insulation. If there is adequate space under the joists, then insulation can easily be installed from below.

• For solid floors the least disruptive option is to lay 5-mm cork tiles.

• New external doors and windows should conform to current Building Regulations in terms of thermal efficiency. The ecological choice for windows is timber framing from a sustainable source.

• Explore the possibility of grants.

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